Shielding film for a light receiving element and optically coupled insulating device

ABSTRACT

A light receiving element includes: a semiconductor layer; an insulating layer; an interconnect layer; and a film. The semiconductor layer includes a light receiving unit configured to convert a signal light incident on the light receiving unit into an electrical signal. The insulating layer is provided on the semiconductor layer. The interconnect layer is provided on the insulating layer. The film is provided on the insulating layer to cover the light receiving unit and be connected to the interconnect layer, the film being made of a metal or a metal nitride.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 14/203,114,filed on Mar. 10, 2014 which is based upon and claims the benefit ofpriority from Japanese Patent Application No. 2013-190503, filed on Sep.13, 2013, the entire contents of each are incorporated herein byreference.

FIELD

Embodiments described herein relate generally a light receiving elementand an optically coupled insulating device.

BACKGROUND

In many industrial electronic devices, communication devices, and thelike, different power supply systems such as an AC power supply system,a DC power supply system, a telephone line system, etc., are disposedinside the same device to transmit an electrical signal.

In such a case, operations can be stable and safety can be ensured byusing an optically coupled insulating device that can transmit theelectrical signal in a state in which the input circuit and the outputcircuit are insulated from each other.

When a high voltage of 1 kV or more is applied between the inputterminal and the output terminal of such an optically coupled insulatingdevice, a noise component may occur in the light receiving element dueto the electrostatic capacitance of an insulating layer between theinput terminal and the output terminal.

Such noise can be reduced by covering the light receiving element withan electromagnetic shield film made of ITO (Indium Tin Oxide), a metalthin film having a mesh configuration, etc. However, it is difficult toimprove the suitability for mass production of such an electromagneticshield film in the manufacturing processes of the light receivingelement.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic plan view of a light receiving element accordingto a first embodiment;

FIG. 2 is a schematic cross-sectional view of the light receivingelement along line A-A of the light receiving element of the firstembodiment;

FIG. 3A is a graph showing the dependence of the transmittance on thethickness of the electromagnetic shield film; and FIG. 3B is a graphshowing the sheet resistance for the reciprocal of the thickness of theelectromagnetic shield film;

FIG. 4 is a schematic cross-sectional view of an optically coupledinsulating device including the light receiving element of the firstembodiment;

FIG. 5A is a schematic view showing a measurement system of theinstantaneous common mode rejection voltage of the optically coupledinsulating device; and FIG. 5B is a waveform diagram showing the changeof the pulse voltage;

FIG. 6 is a schematic cross-sectional view of a light receiving elementaccording to a second embodiment;

FIG. 7 is a schematic cross-sectional view of a modification of thelight receiving element of the second embodiment;

FIGS. 8A to 8F are schematic views showing manufacturing processes ofthe electromagnetic shield film 24 including the metal nitride thinfilm; and

FIGS. 9A to 9F are schematic views showing a modification of themanufacturing processes of the electromagnetic shield film including themetal nitride thin film.

DETAILED DESCRIPTION

In general, according to one embodiment, a light receiving elementincludes: a semiconductor layer; an insulating layer; an interconnectlayer; and a film. The semiconductor layer includes a light receivingunit configured to convert a signal light incident on the lightreceiving unit into an electrical signal. The insulating layer isprovided on the semiconductor layer. The interconnect layer is providedon the insulating layer. The film is provided on the insulating layer tocover the light receiving unit and be connected to the interconnectlayer, the film being made of a metal or a metal nitride.

Embodiments of the invention will now be described with reference to thedrawings.

FIG. 1 is a schematic plan view of a light receiving element accordingto a first embodiment.

The light receiving element 10 includes a semiconductor substrate, asemiconductor layer provided on the semiconductor substrate, a metalinterconnect layer, an insulating layer, and an electromagnetic shieldfilm including a metal nitride thin film or a metal thin film.

The semiconductor layer includes a light receiving unit 20 that convertsa signal light incident on the light receiving unit 20 into anelectrical signal. The length of one side of one unit of the photodiodesincluded in the light receiving unit 20 may be 20 to 200 μm, etc.; andthe units may be disposed in an array configuration or a mosaic arrayconfiguration.

The semiconductor layer may further include a signal processing circuitunit that processes and outputs the electrical signal. The signalprocessing circuit unit includes a TIA (a Transimpedance Amp) 32, acomparator 34, an output circuit 36, a current source 38, an amplifierbias circuit 40, etc. Also, the light receiving element 10 may includeelectrode pad units 42 to 46, etc.

FIG. 2 is a schematic cross-sectional view of the light receivingelement along line A-A of the light receiving element of the firstembodiment.

A semiconductor substrate 70 may be, for example, Si, SiC, etc. Asemiconductor layer 72 that includes Si, etc., is provided on thesemiconductor substrate 70 to include the light receiving unit 20 thatconverts a signal light Lin incident on the light receiving unit 20 intothe electrical signal, and the signal processing circuit unit thatprocesses and outputs the electrical signal. For example, thesemiconductor substrate 70 is the p⁺-type; and an n-type layer isprovided on the semiconductor substrate 70. For example, a p⁺-type layer72 a is formed by ion implantation, etc., from the front surface side toreach the semiconductor substrate 70 to separate the light receivingunit 20 from the signal processing circuit unit.

P-type layers and/or n-type layers are provided in the semiconductorlayer 72 to form the signal processing circuit unit by forming MOStransistors and/or bipolar transistors. The light receiving unit 20includes a high-resistance n-type layer 72 b, a p⁺-type layer 72 c, ann⁺-type layer 72 d, etc. The conductivity types of the regions of thesemiconductor substrate 70 and the semiconductor layer 72 may be theopposite of those of the structure of FIG. 2.

Metal interconnect layers 50 are provided at the upper portion of thesemiconductor layer 72 to be connected respectively to the lightreceiving unit 20 and a signal processing circuit unit 30. In the caseof metal interconnect layers 50 a to 50 d in which a plurality of layersare stacked with an insulating layer 60 interposed, the multiple layersare connected to the layers above and below at prescribed locations ifnecessary.

The insulating layer 60 is provided on the front surface of thesemiconductor layer 72 to fill between the metal interconnect layers 50a to 50 d. The insulating layer 60 may be, for example, a SiO_(x) film,a SiN_(x) film, a low relative dielectric constant (low k) film, etc.

A film (an electromagnetic shield film) 24 that includes a metal nitridethin film or a metal thin film covers a portion of the upper surface orthe entire upper surface of the light receiving unit 20 with theinsulating layer 60 interposed and is grounded via one of the metalinterconnect layers 50. On the other hand, the anode or the cathode (inFIG. 2, the p⁺-type anode) of the light receiving element 10 also isgrounded. Thus, the electromagnetic shield effect can be increased.

FIG. 3A is a graph showing the dependence of the transmittance on thethickness of the electromagnetic shield film; and FIG. 3B is a graphshowing the sheet resistance for the reciprocal of the thickness of theelectromagnetic shield film.

In FIGS. 3A and 3B, the electromagnetic shield film is a metal nitridethin film made of TiN. In FIG. 3A, the vertical axis is thetransmittance (%) of near-infrared light of 770 nm; and the horizontalaxis is the thickness (nm) of the TiN film. The transmittance decreasesto 50% when the film thickness is about 12 nm; and the transmittancedecreases to 10% when the film thickness is about 40 nm.

In FIG. 3B, the vertical axis is the sheet resistance (Ω/sq.); and thehorizontal axis is the reciprocal (1/nm) of the film thickness. Theelectromagnetic shield effect increases as the sheet resistancedecreases. By the evaluation of the common mode noise immunity describedin detail below, the inventors discovered that misoperations due to thenoise occurring due to a pulse voltage slope of 1 kV/μs can be reducedby setting the sheet resistance to be 350 Ω/sq. or less (within theapplicable range shown in FIG. 3B). In the case of TiN, the sheetresistance can be 350 Ω/sq. or less by setting the film thickness to be5 nm or more. In such a case, the electromagnetic shield film can beused in the light receiving element while maintaining a hightransmittance of about 75%.

On the other hand, the sensitivity of the light receiving unit 20decreases when the transmittance is lower than 30%. In other words, itis favorable for the film thickness to be 21 nm or less and thetransmittance to be 30% or more (within the applicable range shown inFIG. 3A). In the case where the film thickness is 21 nm, the sheetresistance can be low, i.e., about 85 Ω/sq.; and a high electromagneticshield effect can be maintained.

The metal nitride thin film is not limited to TiN and may be TaN, ZrN,VN, NbN, etc. At 20° C., the resistivity value is 21.7 μΩ·cm for TiN,135 μΩ·cm for TaN, 13.6 μΩ·cm for ZrN, 200 μ·cm for VN, and 200 μΩ·cmfor NbN. In the case of Si, such metal nitride thin films can be used asa barrier layer that separates the metal interconnect layers from the Sito suppress the diffusion and/or migration of the metal. In other words,it is easy to use the metal nitride thin film in the manufacturingprocesses of the integrated circuit; and the productivity of the lightreceiving element 10 can be increased. Further, a metal thin film of Ti,Ta, W, Co, Ni, Al, Cu, etc., may be used as the electromagnetic shieldfilm 24.

On the other hand, there are structures in which a transparentconductive film such as ITO (Indium Tin Oxide), a polycrystalline Sifilm, a mesh metal film, or the like is used as the electromagneticshield film. In the case where ITO is used, the suitability for massproduction degrades because a dedicated apparatus must be providedoffline because the ITO film formation process is different from themanufacturing processes of the integrated circuit. Moreover, thereliability of ITO is insufficient because film breakage occurs easilyat stepped portions. Further, the pattern dimensional precision and thepatterning reproducibility are insufficient.

In the case of a polycrystalline Si film, the transmittance decreasesradically near visible light; and light absorption of near-infraredlight also occurs. In the case where the thickness is reduced to reducethe light absorption, the sheet resistance becomes high; and thewavelength dependence of the transmittance increases due to the highrefractive index. Further, the film formation process of polycrystallineSi may exceed 500° C.; and it is difficult to form the polycrystallineSi on the metal interconnect layer.

In the case of a mesh metal film, the electromagnetic shield effectdecreases drastically if the openings are too large; and if the mesh isfine, the sensitivity of the light receiving element is reduced becausethe incident light is reflected. Also, the suitability for massproduction of the mesh formation process is insufficient.

Thus, the reliability and suitability for mass production areinsufficient for an electromagnetic shield film made of a transparentconductive film, a polycrystalline Si film, or a mesh metal film.Conversely, it is easy to improve the characteristics and suitabilityfor mass production of an electromagnetic shield film made of a metalnitride thin film.

FIG. 4 is a schematic cross-sectional view of an optically coupledinsulating device including the light receiving element of the firstembodiment.

The optically coupled insulating device (including photocouplers andphotorelays) 80 includes the light receiving element 10 of the firstembodiment and a light emitting element 84 that is electricallyinsulated from the light receiving element 10 and irradiatesnear-infrared light toward the light receiving unit 20. If the lightreceiving element 10 is provided on a signal output unit, i.e., anoutput lead 83, and the light emitting element 84 is provided on asignal input unit, i.e., an input lead 82, an inner resin layer 86 andan outer resin layer 87 may be further provided around the lightemitting element 84 and the light receiving element 10 which oppose eachother. The light receiving element and the light emitting element may beprovided on an insulating substrate and sealed with a resin layer.

FIG. 5A is a schematic view showing a measurement system of theinstantaneous common mode rejection voltage of the optically coupledinsulating device; and FIG. 5B is a waveform diagram showing the changeof the pulse voltage.

In the optically coupled insulating device 80, the input lead 82 (thelight emitting element 84 side) is insulated from the output lead 83(the light receiving element 10 side). Therefore, there is a straycapacitance between the input lead 82 and the output lead 83. When apulse voltage V_(CM) that changes abruptly is applied between the inputand output leads, a displacement current flows; and noise that causesmisoperations occurs in an output voltage Vo of the light receivingelement 10. The instantaneous common mode rejection voltage can beexpressed as the common mode noise immunity (CMR (Common ModeRejection)). In other words, a high CMR means that the noise immunity ishigh.

The CMR is measured as the change of the output voltage Vo of the lightreceiving element 10 when the pulse voltage V_(CM) that changes abruptlyis applied between an input lead 82 a and an output lead 83 a in a statein which a power supply voltage Vcc is supplied. In other words, the CMRis defined by the voltage slope (kV/μs) of the maximum pulse voltageV_(CM) for which the change of the output voltage Vo is not more than aprescribed value. For example, in the case where the electromagneticshield film 24 has a thickness of 21 nm, a CMR that is 10 kV/μs or moreis possible.

FIG. 6 is a schematic cross-sectional view of a light receiving elementaccording to a second embodiment.

The electromagnetic shield film 24 that is made of the TiN thin film,etc., is connected to the output lead (the grounding side) 83 a of theoptically coupled insulating device via the metal interconnect layer 50d. Also, one terminal of the light receiving unit 20 and the groundingterminal of the signal processing circuit unit 30 are connected to theoutput lead 83 a. For example, the input terminal of the TIA amplifier32 that is provided in the front end of the light receiving element 10as shown in FIG. 5A is connected to the other terminal of the lightreceiving unit 20.

In FIG. 6, the insulating layer 60 is interposed and a stray capacitanceCns occurs between the electromagnetic shield film 24 and the cathode(the n-type diffusion layer) of the photodiode included in the lightreceiving unit 20. The stray capacitance Cns is unfavorable because thestray capacitance Cns increases the capacitance of the input terminal ofthe TIA amplifier 32 and causes the frequency characteristics and/or thenoise characteristics to degrade. Therefore, it is favorable for thedistance between the electromagnetic shield film 24 and thesemiconductor layer 72 to be long. For example, the distance is set tobe 1 μm or more. In FIG. 6, the stray capacitance Cns is reduced byproviding the electromagnetic shield film 24 at a position between themetal interconnect layer 50 d, which is the uppermost layer, and themetal interconnect layer that is adjacent to the metal interconnectlayer 50 d.

The misoperations of the optically coupled insulating device 80 forwhich the electromagnetic shield effect is increased by theelectromagnetic shield film 24 can be reduced further by the signalprocessing circuit unit 30 being a differential circuit unit.

FIG. 7 is a schematic cross-sectional view of a modification of thelight receiving element of the second embodiment.

The electromagnetic shield film 24 that is made of TiN, etc., isconnected to the upper surface of a top layer of the plurality ofinterconnect layers 50 by contacting the top layer. The top layer is amost distal layer of the plurality of interconnect layers 50 to thesemiconductor layer. Therefore, it is unnecessary to provide athrough-hole between the metal interconnect layer 50 d and theelectromagnetic shield film 24 in the interior of the insulating layer60; and the stray capacitance Cns can be reduced. The substrate of thelight receiving element 10 is not limited to the p-type substrate andmay be an n-type substrate. Also, the metal interconnect layers 50 maybe a metal such as Al, Cu, Ti, etc.

FIGS. 8A to 8F are schematic views showing manufacturing processes ofthe electromagnetic shield film 24 including the metal nitride thinfilm. Namely, FIG. 8A shows the metal interconnect layer 50 d of theuppermost layer of the multiple metal interconnect layers. In FIGS. 8Ato 8F, the metal interconnect layers that are positioned between thesemiconductor layer and the metal interconnect layer 50 d which is theuppermost layer are not shown. FIG. 8B is a schematic cross-sectionalview of a structure in which a photoresist 90 is patterned; FIG. 8C is aschematic cross-sectional view of a structure in which the metalinterconnect layer is selectively removed; FIG. 8D is a schematiccross-sectional view of a structure in which a metal nitride thin filmis provided; FIG. 8E is a schematic cross-sectional view of a structurein which a photoresist 91 is patterned; and FIG. 8F is a schematiccross-sectional view of a structure in which the metal nitride thin filmis removed from the outer region of the light receiving unit.

In these drawings, the electromagnetic shield film 24 is TiN. Althoughthe electromagnetic shield film 24 is provided on the metal interconnectlayer 50 d of the uppermost layer to contact the metal interconnectlayer 50 d of the uppermost layer, it is unnecessary for the layercontacted by the electromagnetic shield film 24 to be the uppermostlayer.

As shown in FIG. 8A, the metal interconnect layer 50 d is formed abovethe semiconductor layer 72 in which the light receiving unit 20 and thesignal processing circuit unit 30 are provided. Thin barrier layers 51and 52 are provided on two sides of the metal interconnect layer 50 d.In the case where the metal interconnect layers 50 are Al (e.g., havingthicknesses of 200 to 1500 nm), the barrier layers 51 and 52 may be TiN(e.g., having thicknesses of 10 to 30 nm). The barrier layer 51 cansuppress migration of the metal interconnect layer 50 d; and the barrierlayer 52 can prevent reflections in the exposure process.

As shown in FIG. 8B, the photoresist 90 is patterned to make openings toat least the upper portion of the front surface of the light receivingunit 20. Continuing as shown in FIG. 8C, at least the metal interconnectlayer 50 d and the barrier layers 51 and 52 on the two sides of themetal interconnect layer 50 d above the light receiving unit 20 areremoved; and the photoresist 90 also is removed. In these drawings, aportion of the metal interconnect layer 50 d and the barrier layer 52 inthe region around the light receiving unit 20 also is removed.

Then, as shown in FIG. 8D, the electromagnetic shield (TiN) film 24having a thickness of 4 to 20 nm, etc., is formed on a portion of thebarrier layer 52 on the metal interconnect layer 50 d and above thelight receiving unit 20 by sputtering, etc. The insulating layer 60 mayremain above the light receiving unit 20.

Continuing as shown in FIG. 8E, the photoresist 91 is patterned to coverthe region above the light receiving unit 20. The structure of FIG. 8Fis obtained by removing the electromagnetic shield (TiN) film 24 that isin the region not covered with the photoresist 91 by RIE (Reactive IonEtching), etc., and by further removing the photoresist 91. Theelectromagnetic shield film 24 that is above the light receiving unit 20is connected to the metal interconnect layer 50 d via the barrier layer52 remaining at the end portion vicinity of the light receiving unit 20.It is possible to improve the suitability for mass production of thelight receiving element 10 because the electromagnetic shield film 24including TiN is used as the barrier layer of the semiconductor device,etc.

FIGS. 9A to 9F are schematic views showing a modification of themanufacturing processes of the electromagnetic shield film including themetal nitride thin film. Namely, FIG. 9A is a schematic cross-sectionalview of a structure in which a first metal interconnect layer isprovided above the semiconductor layer; FIG. 9B is a schematiccross-sectional view of a structure in which the photoresist 90 ispatterned; FIG. 9C is a schematic cross-sectional view of a structure inwhich the metal interconnect layer 50 d is selectively removed; FIG. 9Dis a schematic cross-sectional view of a structure in which anelectromagnetic shield film is provided; FIG. 9E is a schematiccross-sectional view of a structure in which the photoresist 91 ispatterned; and FIG. 9F is a schematic cross-sectional view of astructure in which the electromagnetic shield film 24 is selectivelyremoved. Similarly to FIGS. 8A to 8F, the metal interconnect layers thatare positioned between the semiconductor layer and the metalinterconnect layer 50 d which is the uppermost layer are not shown.Further, the metal interconnect layer is not limited to being theuppermost layer.

In the case where the electromagnetic shield film 24 that is made of TiNis removed by RIE, etc., the upper portion of the metal interconnectlayer 50 d and the barrier layer 52 under the TiN may be removed withthe TiN. By patterning the photoresist 91 in the modification as shownin FIG. 9E, the metal interconnect layer 50 d, the barrier layers 51 and52, and the TiN are etched by a continuous process using the samephotoresist pattern. Therefore, a portion of the metal interconnectlayer 50 d and the barrier layer 52 is not removed and can be used asthe metal interconnect layer.

Because there is no process to form the TiN thin film on the sidesurface of the metal interconnect layer 50 d, the TiN thin film is notprovided on the side surface of the metal interconnect layer 50 d to beirradiated with the ion beam used in the RIE. Therefore, the yielddecrease due to dust made of the TiN, etc., peeling from the sidesurface and soiling the interior of the RIE apparatus is suppressed.

According to the embodiment, a light receiving element is provided inwhich the effects of the noise are reduced and the suitability for massproduction is good. According to the optically coupled insulating deviceincluding the light receiving element, for example, it is easy for theCMR to be 1 kV/μs or more and for misoperations to be suppressed. Whilecertain embodiments have been described, these embodiments have beenpresented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modification as would fall within the scope andspirit of the inventions.

What is claimed is:
 1. A light receiving element, comprising: asemiconductor layer including a light receiving portion; an insulatinglayer provided on the semiconductor layer; an interconnect layerprovided on the insulating layer; and a film provided on the insulatinglayer to cover the light receiving portion and be connected to theinterconnect layer, wherein the film is a metal or a metal nitrideincluding at least one selected from Ti, Ta, Zr, V and Nb.
 2. Theelement according to claim 1, wherein the interconnect layer includeslayers, a part of the insulating layer being provided between thelayers.
 3. The element according to claim 2, wherein the layers areconnected at a prescribed location.
 4. The element according to claim 2,wherein the film is connected to an upper surface of a top layer of thelayers.
 5. The element according to claim 4, wherein the film isprovided to contact a portion of the upper surface of the top layer. 6.The element according to claim 2, wherein the film is provided tocontact at least one of the layers.
 7. The element according to claim 1,wherein the light receiving unit includes an anode and a cathode, andthe film is connected to the anode or the cathode via the interconnectlayer.
 8. The element according to claim 1, wherein the semiconductorlayer further includes a signal processing circuit unit connected to thelight receiving portion to process and output the electrical signal, andthe film is connected to a ground of the signal processing circuit unitvia the interconnect layer.
 9. The element according to claim 1, whereinthe film has a thickness of not more than 21 nm.
 10. An opticallycoupled insulating device, comprising: a light receiving elementincluding a semiconductor layer, an insulating layer, an interconnectlayer, and a film, the semiconductor layer including a light receivingportion, the insulating layer being provided on the semiconductor layer,the interconnect layer being provided on the insulating layer, the filmbeing provided on the insulating layer to cover the light receivingportion and be connected to the interconnect layer, wherein the film isa metal or a metal nitride including at least one selected from Ti, Ta,Zr, V and Nb; a light emitting element configured to irradiate thesignal light toward the light receiving portion; a signal input unit,the light emitting element being provided on the signal input unit; andan output unit insulated from the signal input unit, the light receivingelement being provided on the output unit.
 11. The device according toclaim 10, wherein the film has a thickness of not more than 21 nm.